A conventional solid state image sensor includes an array of light sensitive pixels. Not all of the area of each pixel is light sensitive. In a complementary metal-oxide semiconductor (CMOS) type image sensor, typically about 50% of the area of each pixel is occupied by opaque, non-sensitive circuitry. Light impinging on the non-sensitive pixel area is not collected, resulting in a loss of sensitivity and degraded performance. This is illustrated schematically in FIG. 1, which shows two adjacent pixels of a sensor array each including a sensitive area 200 surrounded by non-sensitive circuitry 202. Light 204 which strikes the sensitive areas 200 contributes to the sensor output signal. Light 206 which strikes the non-sensitive circuitry 202 is scattered and lost (i.e., not detected).
Such loss of sensitivity can be compensated for by the use of microlenses. As illustrated in FIG. 2, a microlens 208 is associated with each pixel. The microlens 208 covers both the sensitive and non-sensitive areas 200 and 202 of the pixel, collecting light from most of the total pixel area and focussing the collected light onto the sensitive area 200 of the pixel. Accordingly, a greater proportion of the light striking the pixel area is collected and, thus, the sensitivity of each pixel is improved. Typically, the use of microlenses approximately doubles the sensitivity of an equivalent sensor without microlenses. This technique is well known in the art as may be seen, for example, in U.S. Pat. Nos. 4,861,140; 5,677,200; 5,250,798; and 5,614,590.
The arrangement shown in FIG. 2 is a simplification, showing only light rays which are perpendicular to the sensor surface so that the microlenses focus the light rays onto the center of the sensitive area 200 of each pixel. This is true at the optical axis of the image sensor but, for pixels located off-axis, the light rays are non-perpendicular to the sensor surface, as shown in FIG. 3.
Here, because the light rays are non-perpendicular, the area onto which the light rays are focussed is shifted away from the center of the sensitive area 200, but is still entirely within the sensitive area. The size of the shift is dependent on the curvature and refractive index (i.e., the focal length) of the microlens 208, the distance between the microlens and the sensor surface, the primary optics of the imaging system of which the sensor forms a part, and the distance of the pixel from the optical axis.
As long as the light collected by the microlens 208 is focussed entirely within the sensitive area 200 of the pixel, the shift of the focussing spot is not significant. However, if the shift is so extreme that the focussing spot is shifted partly or wholly out of the sensitive pixel area 200, as shown in FIG. 4, the pixel will fail to detect some or all of the light, resulting in a loss of sensitivity known as “vignetting.” This is particularly problematic with CMOS type image sensors which include a greater number of metal layers than, for example, CCD devices. As a result, there is a larger distance between the microlens and the sensor surface, and this increases the shift of the focussing spot.
One attempt to address this problem is to increase the exit pupil of the primary lens system, but this is expensive and not cost effective for CMOS sensors which are generally used in low-cost imaging systems. Another attempt to address the problem of vignetting is disclosed in U.S. Pat. No. 5,610,390, in which the position of the microlens relative to the pixel is adjusted by an amount which varies with the distance of the pixel from the optical axis of the sensor array. This is illustrated in FIG. 5, where the microlenses are shifted to the left by a distance d, as compared with FIG. 4, so that the focussing spots are shifted back into the sensitive areas 200 of the pixels. According to U.S. Pat. No. 5,610,390, the optical axis of each microlens is displaced towards the central axis of the sensor array by an amount which is proportional to the distance between the pixel and the central axis.
In theory, for a given sensor, microlens array and primary lens system, the spacing between adjacent microlenses may be adjusted, and hence their positions varied relative to the pixel array, such that all of the light collected by each microlens is focussed on the center of the associated pixel. However, this solution would require different microlens arrays to suit the parameters of specific imaging systems. Also, the relative microlens shifts required for adjacent pixels are so small that it is impractical to apply the necessary displacements to individual microlenses in the manner disclosed in U.S. Pat. No. 5,610,390.